1. Field of Invention
This invention relates to a toner for use in electrophotography or electrostatic printing. More particularly, the invention provides a toner which overcomes various disadvantages of known toners, such as toner scatter, background fog, and contamination of a fixing roller, while maintaining a high image density and a toner which exhibits satisfactory developing properties, fixing properties and anti-offset properties as well as improved durability and improved image uniformity to provide a high quality image.
2. Description of Related Art
In electrophotography and electrostatic printing, a toner is used for development of an electrostatic latent image. A toner generally comprises ground particles of a prescribed size of a resin composition comprising a binder resin having incorporated therein a colorant, a charge control agent, etc.
Development of an electrostatic latent image is performed by a two-component system using a two-component developer composed of a toner and a magnetic carrier in which a photoreceptor having an electrostatic latent image thereon receives a stroke of a magnetic brush of the developer to form a toner image, or by a one-component system using a one-component developer solely comprising a toner in which a developing sleeve of a thin layer of charged toner particles is formed and the toner is adhered on a photoreceptor having a latent image thereon by flying or stroking under a development electric field to form a toner image on the photoreceptor. The thus formed toner image is then transferred from the photoreceptor surface to paper. The transferred toner image is usually fixed by contact with a fixing heat roller.
In either of the above-described developing systems, the toner must be charged to have a proper charge quantity and always maintain a constant charge quantity on repetition of image formation.
Charging of the toner is generally effected through frictional electrification, i.e., friction of a toner predominantly with a magnetic carrier in the case of the two-component developer or friction of a toner predominantly with a blade in the case of the one-component developer. In either case, friction among toner particles also serves as a great charging factor. Charging characteristics of toners depend to some extent on the binder resin used, but the charge quantity is not stable on repetition of image formation, easily getting too high or too low. This problem has been coped with by incorporating a charge control agent into the toner to stabilize the toner charge quantity. However, since a toner is very delicate in charging, if a resin composition has poor dispersion properties, charging properties greatly vary among individual toner particles even in the presence of a charge control agent. This being the case, toner particles of opposite polarity or toner particles of low charge are produced in an appreciable proportion, causing a scatter of a toner inside an image forming apparatus and background fog. Further, if the charge control agent is localized in the toner, the toner and the charge control agent are deposited and accumulated on a fixing roller, causing contamination of not only the fixing roller but also the transfer paper.
Moreover, the dispersion state of a colorant in the composition not only decides the coloring power but has great influence on charging characteristics of the toner. Poor dispersion of the colorant sometimes causes an abnormal increase in charge quantity or insufficient charging.
To meet demands for high image quality, developing conditions have become increasingly severe. Agglomeration of a toner as well as instability of charge quantity easily cause deterioration of image quality. The dispersion state of additives such as the charge control agent and the colorant influences agglomeration of a toner, sometimes causing apparent fog or white spots (where a toner is lacking in an image area).
In the field of commercial electrophotographic copy, there are applied systems which comprise forming an electrostatic image on a photoreceptor, developing this electrostatic latent image with charged toners, transferring the formed image of the toner onto a paper and the like, heat-fixing the transferred image.
The fixing of a toner image transferred to transfer paper is generally carried out by a thermal fixing process, preferably using a heat roller for greater ease and safety of operation. However, such thermal fixing can cause the off setting of a toner to the roller. Various means for preventing offset have hitherto been proposed. For example, an offset preventing liquid is applied to the fixing roller, or a parting agent is incorporated with the toner.
Another way which has been proposed to prevent offset is by controlling the molecular weight or viscoelasticity (rheological characteristics) of a binder resin of the toner. For example, JP-A-59-214960 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes to use a thermoplastic resin having a complex elastic modulus whose real part (storage of elastic modulus) ranges from 5.times.10.sup.4 to 5.times.10.sup.5 Pa and whose imaginary part (loss of elastic modulus) ranges from 5.times.10.sup.4 to 2.times.10.sup.6 Pa as a binder resin.
Although the '960 proposals are of interest for their effect on storage and loss of elastic modulus in a binder resin, no strict correlation was established between these rheological characteristics and the actual fixing and anti-offset properties. While the proposals are concerned with the binder resin to be used, in actual use the toner still shows a scattering in the range of fixing temperature, the occurrence of offset, depending on the dispersion state of a colorant or a charge control agent, even with a binder resin conforming to the standards of the '960 proposals. Thus, satisfactory results have not yet been obtained.
Even though a toner image may be fixed on transfer paper without the occurrence of offset, cases still occur not infrequently in which unevenness of the surface of the fixed image impairs uniformity of the image area. This results in a local difference in density or gloss, which further results in a nonuniform or unclear appearance. Besides deteriorating the image quality, the unevenness of the fixed image surface causes toner particles to fall off the image area when the copy contacts other objects, such as hands, paper, and desks. Thus hands, clothing, and other sheeting become stained as well as the copy itself .
Further, toner particles having poor impact resistance are apt to become pulverized or release dispersed, thus acting as substances which produce defective particles when the toner is agitated in a developing machine. Production of such defective particles leads to production of spent toner on the carrier or charge-imparting element (e.g., a blade or a sleeve). It can also lead to variation in the charging tendency of a developer, thereby greatly changing the charge quantity of the toner.